should the polygons of a model be squares/rectangles?
The term you are looking for is "quads".  Tris have 3 vertices and n-gons have more than 4.

Quads are better in general for modelling in Blender, particularly if you are planning to animate.

Quads allow for clean straight lines.  This is SUPER important.  Loop cuts can easily be introduced, selection is easier, UV unwrapping is easier, beveling can be done. 

The list goes on and on.

You will learn how to create quad based topology.  It's worth the effort and a great foundation for learning Blender.  So good for you!  You are learning one of the most important early lessons in modeling.